1. Field of the Invention
The invention relates to frequency selective limiters (FSLs) and in particular to FSLs having a temperature and frequency compensated magnetic bias field which results in improved performance over its bandwidth.
2. Description of the Prior Art
Presently manufactured frequency selective limiters 10 (FSLs), shown in FIG. 1, employ a YIG element 12 formed of a pair of single crystal YIG slabs 14 with a conductor 16 sandwiched therebetween. An exemplary YIG structure has a 111 lattice that has been grown in a liquid epitaxy furnace on a GGG substrate 18. The magnetic resonant line width of YIG is about 1 oerstead or less. A static bias field H is applied in the plane of the conductor an angle of 90.degree. thereto.
The static magnetic field H is provided by a pair of opposed permanent magnets 20 placed on either side of the YIG element 12. Typically, the strength of the field produced by the magnets 20 decreases with increasing temperature. The strength of the magnetic field H required to achieve a particular performance level at various temperatures within a given temperature range of interest may be quite different from the field actually produced by the magnet 20. Indeed, there may be only one or two values where the required field H for the particular temperature is correct. As a result, the performance of the YIG element 12 may degrade with changing temperature.
FSLs are typically employed in a bandwidth from about 2.5 to about 5.5 GHz. In this bandwidth, it is normally desirable to develop flat limiting characteristic between the two frequencies (see FIG. 2). Typically, however, limiting rolls off at the lower frequencies. As the field strength is increased however, the characteristic roll-off moves to the left in FIG. 2 so that limiting is generally flat over the bandwidth. If the magnetic field strength is increased beyond the certain limit, the low frequency roll-off tends to move to the right as a result of volume wave propagation. Thus, there is only a small range of field values which results in advantageous limiting.
The limiting illustrated in FIG. 2 results at a specific temperature and magnetic field strength. Accordingly, as the temperature changes, the magnetic field strength changes causing a variation in the limiting characteristic.
In FIG. 3 curve A illustrates the ideal relationship of magnetic field strength versus temperature for a YIG element. In addition, the tolerance around the ideal is illustrated in dotted line. The curve shows that over a given exemplary temperature range from about -50.degree. C. to about 85.degree. C. the required field strength decreases with increasing temperature.
The curves B-E of magnetic field strength versus temperature, for various charging levels, of a typical permanent magnet are also shown in FIG. 3. The actual slope of each curve B-E is dependent upon a number of factors, but is primarily dependent on the initial magnetization or charge of the magnet. For example, a magnet with a low charge produces a curve B exhibiting low field strength which is much shallower than the ideal A. As the charge on the magnet is increased, the strength of the field produced by the magnet increases absolutely over the temperature range and the slope of the curve increases until it reaches the slope of the ideal (curve C), more highly charged magnets exhibit a steeper slope (curves D-E). Unfortunately, as the charge is increased, the field strength increases to such a level that the limiting characteristic is degraded, i.e. moved to the right in FIG. 2 thereby degrading the performance of the FSL. Also, no means has been formed to tailor the curve to all temperatures.
It has also been found that the low frequency roll-off of the attenuation curve in FIG. 2 can be advantageously affected by rotating the biasing field by a small amount a set forth in copending patent application Ser. No. 658,498, filed Feb. 21, 1991 entitled "Frequency Selective Limiter With Flat Limiting Response" by McGann et al. and assigned to Westinghouse Electric Corporation the assignee herein, the teachings of which are incorporated hereby reference. In that arrangement, the field is rotated with respect to the conductor by physically orienting the YIG and conductor carried thereby at an angle with respect to the field or by providing a zigzag conductor on the YIG film. While effective, the solutions set forth in the application create volume efficiency reductions and manufacturing difficulties which need improvement.